N-p-n Structure Promises Efficient GaN LED Performance At High Currents
It is imperative to read the small print when judging the performance of high-brightness GaN LED chips. That s because efficacy, the metric most frequently used to rate device operation, is flattered by the 20 mA drive current tests that are often quoted. At the far higher drive currents that are actually used for operating a chip in lighting applications, efficacies are significantly lower and at 1 A they are typically only half of the 20 mA value (see figure 1 for examples from two LED manufacturers).
This decrease in LED efficacy with increasing drive current is clearly an issue for chip manufacturers. Its origins are not well understood but, if uncovered, could provide a starting point for boosting the performance at operating conditions required for general illumination.
The misconception is that the quenching of efficacy at higher currents is caused by heating of the active region, say Igor Rozhansky and Dmitry Zakheim, researchers studying LED performance at the Ioffe Physico-Technical Institute of the Russian Academy of Sciences in St Petersburg. However, this has been ruled out by comparing the external quantum efficiency (EQE) of devices driven continuously with those operating in pulsed mode, which run cooler.
Rozhansky says that another popular theory for explaining reduced performance at higher currents is associated with the filling of localized states in the InGaN active region. This argument assumes that at low currents the electrons and holes are confined to areas where there are fluctuations in indium composition. This suppresses transport to non-radiative recombination centers. At higher currents these localized states are already full, which increases the probability that the additional electrons and holes reach, and are trapped by non-radiative recombination centers that prevent luminescence.
According to Rozhansky, this explanation predicts that InGaN LED efficacy is strongly influenced by the quality of the active region. However, measuring the efficacy of a range of LEDs with different active regions at high current densities has shown that this is not true.
Instead, Rozhansky and his colleague think that the explanation for the efficacy drop at higher currents is a decrease in the effectiveness of the electron-blocking barrier. This barrier, which features in many LEDs, boosts emission by preventing electrons from escaping into the p-type region. "In AlGaAs layers they are effective," he explains, "but in nitride-based LEDs it is a different situation – the piezoelectric field that is inherent in the AlGaN current blocking layer substantially reduces the barrier efficiency at high pumping."
Rozhansky and Zakheim have reached these conclusions by using a "drift-diffusion" model to calculate the charge transport in an AlGaInN LED (see I V Rozhansky et al. 2006 Semiconductors 40 861). The modeling has led them to a new LED design with an n-p-n heterostructure that is expected to maintain its efficacy at high drive currents (see figure 2). In this design, the active region is shifted to the LED s p-side and emission is controlled by the injection of electrons instead of holes. LEDs made with this design have backed the theory, bucking the trend of a decreasing EQE at high current densities.
Unfortunately, the on-wafer EQE of the n-p-n structures is typically only 1%, but Rozhansky believes that there are no fundamental reasons why improvements can t be made. The low efficiency is partly caused by a diffusion of the magnesium dopant from the p-type region to the active layer, but this can be addressed by adjusting the doping profile near the active region. Poor quality material, due to growth on magnesium-doped GaN, could also be an issue but it can be dealt with by growing magnesium-doped GaN at higher temperatures, says Rozhansky: "In conventional structures the active region would be destroyed by higher temperatures, but in the n-p-n structures this problem does not exist, so one can obtain much better quality."